Joanna Lumley goes on the trip of a lifetime - exploring the longest river in the world, the River Nile, from sea to source. Surprisingly, much of the journey’s first leg is by train. She’s following the world’s longest river from the sea to the source – 4,000 miles away. “It would take us for ever to do it by boat,” she explains, so politely it would be rude to argue this was the point.